1. Field of the Invention
The present invention relates to a pump system having a pump, including a centrifugal pump; and more particularly to a method and apparatus for pump protection without the use of traditional sensors.
2. Brief Description of Related Art
Other similar devices and their shortcomings are as follows:
U.S. Pat. No. 7,080,508 discloses a method and apparatus for torque controlled pump protection with mechanical loss compensation,” which is hereby incorporated by reference, and which provides control logic that utilizes the direct feedback of torque (or power) and speed to identify undesirable operating conditions and provide the appropriate operating response to protect the driven equipment (centrifugal pump) from damage. The logic can be imbedded in a variable speed drive or Programmable Logic Controller (PLC). However, this technique may be limited to pumps with constantly rising power curves from a closed valve condition. These pumps typically have a specific speed of 2000 and under. This method requires the manual input of power losses which do not factor according to the affinity laws to maintain accuracy over a wide operating speed range.
Moreover, the following devices are known and all fail to include logic that differentiates undesirable operating conditions to control the pump appropriately for each condition without the use of traditional sensors and/or auxiliary controls.
U.S. Pat. No. 6,591,697 discloses a technique for determining pump flow rates using motor torque measurements that provides methodology which explains the relationship of torque and speed versus pump flow rate and the ability to regulate pump flow using a Variable Frequency Drive (VFD) to adjust centrifugal pump speed. However, this device fails to include logic that would provide for protection against undesirable operating conditions. The device utilizes calibrated speed vs. torque curves which are application specific to obtain flow thereby reducing flexibility during field setup.
U.S. Pat. No. 6,464,464 B2, issued to the assignee of the present patent application, discloses a method and apparatus for controlling a pump system that provides a control and pump protection algorithm which uses a VFD to regulate flow, pressure or speed of a centrifugal pump. However, this device requires the use of instrumentation which adds cost and complexity to the drive system, a potential failure point, and unnecessary cost.
Another known device, PMP 25, by Load Controls, Inc. (Sturbridge, Mass.), provides pump protection by observing the motor amperage draw and speed and then correlating the resulting power reading to various operating conditions (e.g. dry running, closed valve condition). (See U.S. Pat. Nos. 5,930,092 and 5,754,421.) However, the Load Controls product is suitable only for constant speed applications and fails to provide control differentiation for various conditions; protective settings result in only “tripping” or shutting off of the motor.
U.S. Pat. No. 6,715,996 B2 discloses a method for the operation of a centrifugal pump that provides methodology which samples the pump power at a closed valve condition for two speeds, determines parasitic losses and calculates an adjusted power at other frequencies to determine if a condition exists which would lead to a malfunction of the motor. However, this technique only protects against zero flow condition it does not include logic to detect a minimum flow condition (flow too low) or runout condition (flow too high) nor can it distinguish between a no demand condition or dry run condition.
PCT WO 2005/064167 A1 discloses a quantitative measurement technique that provides methodology which uses a calibrated power/differential pressure curve vs. flow vs. speed. The calibrated data is stored and compared to current values in order to determine pump flow. However, this technique fails to include logic that would provide for protection against undesirable operating conditions. It also utilizes calibration curves for power/Δ pressure vs. flow at several speeds which are stored in the evaluation device. This method requires application specific data to obtain flow thereby reducing flexibility during field setup.
A product by ABB Industry Oy (Helsinki, Finland) provides a variable frequency drive (VFD) having parameters that allow maximum and minimum torque values to be configured to prevent the load driver (motor) from operating outside of these parameters. However, the ABB drive does not provide logic for interpreting different operating conditions, nor does it allow for scaling of centrifugal loads, such as pumps or take into account mechanical losses in small pumps at reduced speed.
A variable frequency drive system can be configured to utilize flow or pressure switches to identify undesired operating conditions. However, the use of additional process switches adds cost and complexity to the drive system, a potential failure point, and unnecessary cost.
Furthermore, the following patents were developed in a patentability search conducted in relation to the present invention. Below is a brief summary thereof:
United States Publication no. 2004/0064292 discloses a deep well centrifugal pump required to maintain an optimum level. It uses torque and speed data to calculate input power to the pump and uses pump affinity laws to adjust power to rated speed and determines a rated flow based on published pump data. It uses affinity law data and published performance to determine pump head, efficiency and minimum required suction head. The exact calculation method is not presented; it is shown only as flow as a function of power and head, and efficiency and suction head as a function of flow. The method simply calculates power and adjusts it for rated speed and determines flow from published performance data based on the affinity laws. Although widely used in the pump industry, affinity corrections to pump performance are not always accurate.
Although United States Publication no. 2004/0064292 discloses a control system for centrifugal pumps there is no tuning or calibration method involved. This method would require actual pump test data be used or risk introducing significant error. U.S. Pat. No. 6,709,241, which is issued to the assignee of the present application, discloses a technique that requires four sensors plus the input of actual performance data at several speeds in the variable frequency drive. It uses a flow sensor (external flowmeter) to compare actual flow to a threshold value for minimum flow but cannot distinguish between a minimum flow condition, a closed valve condition, a dry run condition or a runout condition.
United States Publication no. 2005/0123408 discloses a self calibration process to determine the minimum speed for which the pump pressure has increased by one increment. It is not used to calibrate power. The dry run protection is based on a comparison of an actual current reading to a threshold value for current. The threshold value is based on one operating speed.
U.S. Pat. Nos. 4,468,219 and 4,795,314 and United States Publication no. US2002/0141875 disclose peristaltic pumps or positive displacement pumps which behave very differently than centrifugal loads with respect to torque and speed.
U.S. Pat. No. 6,783,328 and United States Publication no. 2002/0150476 disclose techniques which require sensors to monitor flow or pressure to compare a setpoint value to a threshold value. If exceeded, the speed is lowered to bring the setpoint below the threshold value.
U.S. Pat. No. 4,650,633 discloses a method that restricts flow to the pump to prevent cavitation based on sensors which detect liquid temperature and pressure at the pump inlet.
Based on an understanding and appreciation of the known prior art discussed above, there is a need in the industry for a technique that provides protection for centrifugal pumps without the use of traditional sensors which can differentiate between dangerous operating conditions (e.g. dry running, minimum flow and runout) and/or conditions where transient conditions (e.g. closed valve operation) may occur and the protection can be revoked once the condition clears.